1. Field of the Invention
The present invention relates to a high strength steel sheet suitable for automotive chassis parts or the like and, more particularly, to a high strength hot rolled steel sheet having tensile strength of not lower than 550 MPa, and to a method for manufacturing the same.
2.Description of Related Arts
From the viewpoint of increased fuel economy leading to environmental protection, hot rolled steel sheets for automobiles, having higher tensile strength and smaller thickness, have been developed. In particular, higher tensile strength and smaller thickness have been demanded for hot rolled steel sheets used for automotive chassis parts such as a suspension arm, to which a great effect of higher tensile strength and smaller thickness is obtained. On the other hand, since the automotive chassis parts have an intricate shape, the hot rolled steel sheet used for them is required to have high elongation and excellent stretch flangeability as well as high tensile strength.
Conventionally, various types of high strength hot rolled steel sheets used for automotive chassis parts have been proposed. For example, JP-A-4-329848 has disclosed a dual phase steel sheet having excellent fatigue property and stretch flangeability, which consists of a ferrite structure and a second phase (pearlite, bainite, martensite, retained austenite, etc.). However, sufficient stretch flangeability cannot be obtained because of the existence of the hard second phase.
JP-A-6-172924 has proposed a steel sheet having excellent stretch flangeability, which has a bainitic ferrite structure with high dislocation density. However, sufficient elongation cannot be obtained because of the existence of the bainitic ferrite structure with high dislocation density.
JP-A-6-200351 has proposed a steel sheet, consisting mainly of a polygonal ferrite structure, which has excellent stretch flangeability and is provided with high tensile strength by utilizing precipitation strengthening due to TiC or solid solution strengthening. However, high elongation and excellent stretch flangeability cannot be obtained steadily because much Ti must be added and thus coarse precipitates are yielded easily.
JP-A-7-11382 has proposed a steel sheet having excellent stretch flangeability, which has an acicular ferrite structure in which fine TiC or NbC is precipitated. However, sufficient elongation cannot be obtained because of the existence of the acicular ferrite structure with high dislocation density.
JP-A-11-152544 has proposed a steel sheet in which the grain size of ferrite is controlled to be as fine as 2 xcexcm or smaller by adding Ti, Nb, V and Mo. However, high elongation cannot be obtained because of the fine grain size of ferrite of 2 xcexcm or smaller.
An object of the present invention is to provide a high strength hot rolled steel sheet having high tensile strength of not lower than 550 MPa, high elongation, and excellent stretch flangeability, which is suitable for intricately shaped automotive chassis parts and a method for manufacturing the same.
The object of the present invention can be achieved by a high strength hot rolled steel sheet containing 0.15% or less C, 0.02 to 0.35% Ti, and 0.05 to 0.7% Mo by weight percentage and consisting essentially of a matrix of ferrite structure single phase and fine precipitates with a grain size smaller than 10 nm dispersed in the matrix. More concretely, the object thereof can be achieved by the following high strength hot rolled steel sheets.
(1) A high strength hot rolled steel sheet which consists essentially of 0.06% or less C, 0.5% or less Si, 0.5 to 2.0% Mn, 0.06% or less P, 0.005% or less S, 0.1% or less Al, 0.006% or less N, 0.02 to 0.10% Ti, 0.05 to 0.6% Mo by weight percentage, and the balance being Fe, wherein fine precipitates with a grain size smaller than 10 nm are dispersed in a matrix of ferrite structure single phase at a number per unit volume of 5xc3x97104/xcexcm3 or higher.
(2) A high strength hot rolled steel sheet which consists essentially of 0.06% or less C, 0.5% or less Si, 0.5 to 2.0% Mn, 0.06% or less P, 0.005% or less S, 0.1% or less Al, 0.006% or less N, 0.02 to 0.10% Ti, 0.05 to 0.6% Mo, at least one element selected from 0.08% or less Nb and 0.15% or less V by weight percentage, and the balance being Fe, wherein fine precipitates with a grain size smaller than 10 nm are dispersed in a matrix of ferrite structure single phase at a number per unit volume of 5xc3x97104/xcexcm3 or higher.
(3) A high strength hot rolled steel sheet which consists essentially of more than 0.06% and 0.15% or less C, 0.5% or less Si, 0.5 to 2.0% Mn, 0.06% or less P, 0.005% or less S, 0.1% or less Al, 0.006% or less N, more than 0.10% and 0.35% or less Ti, 0.3 to 0.7% Mo by weight percentage, and the balance being Fe, wherein fine precipitates with a grain size smaller than 10 nm are dispersed in a matrix of ferrite structure single phase at a number per unit volume of 5xc3x97104/xcexcm3 or higher.
The above-described high strength hot rolled steel sheets can be manufactured, for example, by a method comprising the steps of: producing a hot rolled steel sheet by hot rolling a steel slab having the above-described compositions at a temperature of Ar3 transmission point or higher; and coiling the hot rolled steel sheet at a temperature of 550 to 700xc2x0 C.